Low thermal mass, plate surface heating unit

ABSTRACT

A light weight solid plate surface heating unit of low thermal mass having a top plate of high thermal conductive material for distributing the heat rapidly and uniformly over the plate. The top plate is of composite sheet material with a center core selected from the group of high thermal conductivity materials such as copper, silver and aluminum, and an outer skin selected from the group of corrosion and oxidation resisting materials such as stainless steel, nickel and chromium. There is a heating means for the plate in the form of a metal sheathed electrical resistance heating element of coiled configuration adapted to be held against the underside of the plate. It is important that the top plate remain as flat as possible at all times so as to reduce the formation of gaps between the surface of the top plate and the bottom surface of cooking utensils carried thereon. Reinforcing means are provided for the top plate to prevent warpage due to thermal stresses. A thermal reflecting member is positioned beneath the heating unit to redirect the heat in an upward direction. The heating unit may be held in an opening in the cooktop by providing an adjustable tension member joining the reinforcing member of the heating unit with the reflecting member such that the edge of the opening in the cooktop is clamped by the interaction of the top plate and the reflecting member.

United States Patent 72] Inventor Bohdan Hurko Louisville, Ky. [21]Appl. No. 820,904 [22] Filed May 1,1969 [45] Patented Mar. 9, 1971 [73]Assignee General Electric Company [54] LOW THERMAL MASS, PLATE SURFACEHEATING UNIT 14 Claims, 8 Drawing Figs.

[52] US. Cl. 219/464, 126/400, 219/457, 219/461, 219/462, 219/463,219/530 [51] Int. Cl 1105b 3/68 [50] Field ofSearch 219/461,

[56] References Cited UNITED STATES PATENTS 1,998,764 4/1935 Fordan eta1 219/457 2,400,508 5/ 1946 Hermann 126/215 2,691,717 10/1954 Huck219/462 2,727,133 12/1955 Scofield.... 219/463 2,841,137 7/1958 Chace126/390 3,095,498 6/1963 Foster 219/441 3,196,253 7/1965 Jepson et a1.219/436 3,445,630 .5/1969 Ulam 219/438 3,446,392 5/1969 Lescure l26/390XPrimary Examinerg- Mayewski, Volodymyr Y. and Harold BroomeAtt0rneys-Richard L. Caslin, Harry'F. Manbeck, Jr., Joseph B. F orman,Oscar B. Waddell and Frank L. Neuhauser ABSTRACT: A light weight solidplate surface heating unit of low thermal mass having a top plate ofhigh thermal conductive material for distributing the heat rapidly anduniformly over the plate. The top plate is of composite sheet materialwith a center core selected from the group of high thermal conductivitymaterials such as copper, silver and aluminum, and an outer skinselected from the group of corrosion and oxidation resisting materialssuch as stainless steel, nickel and chromium. There is a heating meansfor the plate in the form of a metal sheathed electrical resistanceheating element of coiled configuration adapted to be held against theunderside of the plate. It is important that the top plate remain asflat as possible at all times so as to reduce the formation of gapsbetween the surface of the top plate and the bottom surface of cookingutensils carried thereon. Reinforcing means are provided for the topplate to prevent warpage clue to thermal stresses. A thermal reflectingmember is positioned beneath the heating unit to redirect the heat in anupward direction. The heating unit may be held in an opening in thecooktop by providing an adjustable tension member joining thereinforcing member of the heating unit with the reflecting member suchthat the edge of the opening in thecooktop is clamped by the interactionof the top plate and the reflecting member.

Pmlamzum 9m: 3,569,672

SHEET 1 U? 2 INVENTOR. BOHDAN HURKO ms ATTORNEY PATENTED "AR 9 |97| MEI2 OF 2 SPIRAL. COlLED Sous PLATE (PR\0R ART) TIME To Bou \QT H2O BMW.

TIME To Bou. \cn' HzO FEGE'B INVENTOR. BOHDAN HURKO Wxm ( PR\OR ART)"III'I FIG. 4-A

F'IG. S-A

H \S ATTORNEY LOW THERMAL MASS, PLATE SURFACE HEATING UNlT BACKGROUND OFTHE INVENTION The average housewife today has become less interested inspending a great deal of her time housecleaning, as there are so manymore challenging opportunities for an intelligent person to occupy hertime outside the home for instance in civic, social or career work.home, there is an ever increasing interest in obtaining modern householdconveniences for simplifying the housecleaning chores. Particularreference is made here to domestic ranges. As to the ease of cleaningthe oven of a range, there has been widespread acceptance of thepyrolytic, self-cleaning oven such as is taught in US. Pat. No.3,121,158 of Bohdan l-lurko, the present inventor. The simple solutionof the age-old problem of maintaining a sparkling clean oven has turnedthe attention of the housewife to the ease of cleaning the cooktop.

A standard electric cooktop is usually provided with a plurality ofmetal sheathed electrical resistance heating elements which are eachwound in a spiral coil and positioned in an opening formed in thecooktop. Each heating element is adapted to support a cooking utensilthereon. These metal sheathed heating elements are self-cleaned of foodsoil due to the high temperatures of about 1200" F. they are permittedto reach once they are energized, but it is'possible for spillovers todrain through the heating element and accumulate in a collecting panlocated beneath the cooktop.

Solid plate surface heating units have been developed prior to this timebut they have not gained general acceptance in this country, because asa general rule they were of heavy cast construction having a relativelyhigh thermal mass which renders them slow to heat up and slow to cooldown as compared with the widely used metal sheathed heating elements.

The principal object of the present invention is to provide a lowthermal mass, solid plate surface heating unit where the plate is madeof a high thermal conductive material for obtaining uniform temperaturedistribution, where the rate of heatup and cool-down is equal to orbetter than the results obtained of a metal sheathed electricalresistance heating element.

A further object of the present invention is to provide a solid platesurface heating unit of low thermal mass by using a top plate ofcomposite sheet material that has a high thermal conductivity on theorder of copper yet is strong and resistant to corrosion and oxidation.

A further object of the present invention is to provide a solid platesurface heating unit of the class described with the provision of adurable, long life heating means in the'form of a metal sheathedresistance heating element of about half the size of a standard metalsheathed unit of comparable wattage.

A further object of the present invention is to provide a solid platesurface heating unit of the class described with a reinforcing means toinsure against warpage of the top plate.

A still further object of the present invention is to provide a solidplate surface heating unit of the class described with an underlyingreflector member and adjustable tension means for clamping the heatingunit and reflector member in an opening in a cooktop.

SUMMARY OF THE INVENTION The present invention, in accordance with oneform thereof, relates to a low thermal mass, solid plate surface heatingunit having a top plate of high thermal conductive material on the orderof copper, where the top plate is of light weight composite sheetmaterial having an outer skin of high strength on the order of stainlesssteel that has resistance to corrosion and oxidation. A reliable andlong life heating means is positioned beneath the top plate and indirect thermal relationship with the underside thereof. Reinforcingmeans are provided for the plate to prevent warpage due to uneventemperature distribution. An adjustable tension means is furnishedbeneath the unit for clamping the unit in an opening in a cooktop.

BRIEF DESCRIPTION OF THE DRAWINGS My invention will be better understoodfrom the following description taken in conjunction with theaccompanying drawings and its scope will be pointed out in the appendedclaims.

FIG. I is a bottom plan view of a light weight solid plate surfaceheating unit embodying the present invention, but minus its reflectormember, showing a spiral coiled metal sheathed electrical resistanceeating element fastened along its length to the underside of a top plateof high thermal conductive material. t

FIG. 2 is a fragmentary cross-sectional elevational view throughone-half of the surface unit of FIG. 1 taken on the line 2-2 thereof,except that the unit is complete with a bottom reflector pan, and theunit is shown mounted in an opening in a cooktop by means of anadjustable tension member at the center of the unit.

FIG. 3A is a combined illustration and performance chart of a 1250 watt,solid plate surface heating unit of the present invention when boilingwater in a small diameter pan.

FIG. 3B is a time-temperature graph for the solid plate heating unit ofFIG. 34 showing the time it takes to boil one quart of water as 8.5minutes.

FIG. 4 A is a combined illustration and performance chart similar toFIG. 3A of a standard or prior art 1250 watt, spiral coiled, metalsheathed resistance heating element when boiling water in a smalldiameter pan.

FIG. 4B is a time-temperature graph for the spiral coiled heatingelement of FIG. 4A showing the time it takes to boil one quart of wateras 13 minutes.

FIG. 5 is a bottom plan vie of a second modification of a solid platesurface heating unit embodying the present invention showing a differentshape of coiled, metal sheathed resistance heating element generally inthe shape of a four-leaf clover, there being a combined reinforcingmember and reflector pan adapted to underlie the heating element and tobe fastened to the underside of the top plate and sealed at itsperiphery therewith so that the area between the plate and the reflectorpan can be evacuated to form a vacuum pressure therein.

FIG. 6 is a cross-sectional elevational view of the second modificationof the present invention taken on the line of 6-6 of FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning now to a considerationof the drawings and in particular to FIG. 2 there is shown across-sectional elevational view of one-half of a solid plate surfaceheating unit 10 embodying the present invention having a top plate 12formed of thin composite sheet material of high thermal conductivematerial with a center core 14 of material such as copper, silver oraluminum for distributing the heat rapidly over the entire plate so asto obtain generally uniform temperature distribution. Copper has verylow strength at temperatures ranging in the vicinity of 1400" F., andalso it oxidizes very readily. Since this copper sheet 14 is of smallthickness, on the order of 0.040 inches, it would tend to warp easilyunder normal use conditions due to thermal stresses caused by uneventemperature distribution, and also due to the high temperatures to whichit is exposed. Even if the top plate is ideally designed and allportions of the plate would operate at the same temperature duringno-load condition or when the cooking utensil used thereon has aperfectly flat bottom, such top plate would develop thermal stresseswhen a cooking utensil is used thereon having a warped bottom ofirregular surface or one with either a convex or a concave bottomsurface. If the bottom wall of the cooking utensil is warped badly theremight only be a few contact points between the utensil and the topheating plate. These contact points would act as heat sink areas andwould tend to create relatively cool spots on the top heating plate. Theremainder of the bottom wall of the utensil would act as a reflectormember redirecting the heat received from the plate back toward theplate and thereby creating hot spots.

for example has a thermal conductivity of 2700 B.t.u./sq.ft.-

hr. F./in. as compared with 96 B.t.u./ft. -hr. F./in. for 309 stainlesssteel. The copper core 14 is sandwiched between two thin, integralstainless steel skins and 16, each of a thickness of about 0.016 inches.The stainless steel skins protect the copper against corrosion andoxidation. In order to avoid exposure of the copper on the peripheraledge of the plate, the two stainless steel skins 15 and 16 will besealed over the edge with a pinching action. Because of the high heatdiffusivity of the copper core 14 a relatively short heating element 18may be used with only three turns or coils. Careful tests have shownthat such a unit has only a 40 F. temperature gradient as compared withabout 270 F. for cold rolled steel plate having a much longer heatingelement. The copper layer diffuses the heat across the plate surface,thus reducing the thermal stresses considerably. The stainless steelskins 15 and 16, being on the outer surface of the plate providestrength to the plate. Such a plate would have only slightly lowerstrength than a solid stainless steel plate of a similar thickness. Thecomposite plate resists warpage because it combines a high strength withhigh heat diffusivity, which no single material plate can provide.

The thin composite sheet material 12 of central copper core 14 and twoouter stainless steel skins 15 and 16 may be formed of individual sheetswhich are area welded together as by a process of explosive weldingwhich causes a bonding of the metal sheets along their mating surfaces.

Looking at FIG. 1, which is a bottom plan view of the solid plateheating unit of FIG. 2 but with the bottom reflector pan removed, thenature of the metal sheathed electrical resistance heating element 18 iseasily discernible. As is well understood by those skilled in this art,such a metal sheathed heating element 18 would include a centralresistance wire 11 of helical formation that is inserted into a metaltube 13 and the tube is filled with a suitable electrical insulatingmaterial and thermal conductive material 17, such as magnesium oxide orthe like. Notice in FIG. 2 that the heating element 18 is shown in aflattened condition which of course would have the effect of flatteningthe helical resistance wire 11. In order to increase the strength of thetop plate 12, the heating element 18 is fastened along the lengththereof to the underside of the top plate 12 as by brazing.

Another method of strengthening the top plate 12 is to form itsperipheral edge with a downturned flange 20 which may either be slightlytapered downwardly as shown in FIG. 2 or which may be of generallyvertical configuration. Again to ensure the rigidity of the top plate12, a reinforcing member 22 is positioned beneath the plate and fastenedto the underside thereof to increase its depth. This reinforcing member22 may be of many different configurations, as there is no one way todesign it. The one shown has a central member 24 in the shape of ahollow cylinder, and extending radially therefrom is shown a series offive struts 25 in the form of edgewise vertical plates which underliethe top plate 12 and extend to the peripheral flange 20 thereof. Thesestruts 25 are each undercut slightly along the top edge so as toaccommodate the turns of the flattened heating element 18 which extendtherethrough. Thus the top edge of these struts is fastened, as bybrazing, to the underside of the top plate 12.

FIG 2 shows an assembly view of the solid plate surface heating unit 10where there is a supporting cooktop 27 having a circular opening 28 forthe heating unit. Notice that the cooktop, in the area surrounding theperipheral edge of the opening 28, is shown slightly depressed as at 30,the purpose being to recess the top plate 12 slightly so as tofacilitate the ease of sliding a cooking utensil on and off of theheating unit. Moreover, the recess would be easy to clean of food soil.The top plate 12 could be coated with a high temperature porcelainenamel that would match or compliment the porcelain enamel finish of thecooktop 27. It is also well to provide a liquid sealing means betweenthe cooktop 27 and the top plate 12 of the surface unit 10 to preventspillovers from passing through the cooktop opening 28 and into the areabeneath the cooktop where the electrical lead wires (not shown) for theheating element 18 would be strung. Thus, I provide a small asbestosgasket 32 beneath the edge of the inclined flange 20 of the top plate 12and it serves a dual function of a liquid sealing means as well as athermal break to reduce the conduction of heat from the plate 12 to thecooktop 27 and to render the heating unit more effective for itsintended purpose.

For improved results it is best to provide a reflector member 35 beneaththe heating unit 10 so as to redirect the heat from the heating element18 in an upward direction. This reflector 35 may also be used as a partof a holddown means for the heating unit. Notice that the reflector 35is of a size larger than the opening 28 in the cooktop so that itunderlies the peripheral edge of the opening. Rather than having thereflector 35 bear directly against the underside of the cooktop 27, aseries of widely spaced clip members 37 are fastened to the undersideof'the edge of the opening 28 for spacing the reflector 35 away from theunderside of the cooktop so as to reduce the conductive path and providea thermal break between the heating unit and its reflector.

.I-bolt adjustable tension member in the form of an inverted .I-bolt 39is adapted to be connected between the reinforcing member 22 and thereflector 35 as is best seen in FIG. 2. The central member 24 of thereinforcing member 22 is provided with a diagonal, vertical plate 40having an aperture 41 through which the head of the J-bolt 39 isinserted. The reflector 35 has a central opening 43 for receiving thelower end of the J-bolt 39 therethrough. The lower end of the bolt has athreaded portion for receiving an adjusting nut 45 thereon.

Clearly this solid plate surface heating unit 10 can only be fastened inthe opening 28 of the cooktop 27 from the underside thereof. If it isnot felt important to mechanically hold down the surface heating unit10, the reflector 35 could merely be rested on the bottom of a rough-inbox (not shown) which is a standard element of a dropin cooktop forenclosing the electrical lead wires and terminals of the heating element18 and in some cases the control switches (not shown) for governing therate of heating of the heating element 18.

One of the primary advantages of the solid plate surface heating unit ofthe present invention is that its performance is superior to the presentday spiral coiled heaters for the smaller diameter pans. The heatreadily flows laterally through the top plate from the outer area notcovered by the pan to the central area covered by the pan bottom. Thisis as illustrated in FIG. 3A taken in comparison with a standard spiralcoiled heating element 65 of FIG. 4A. With the solid plate unit 10 ofFIG. 3A, the outer uncovered area runs relatively cool at about 14.8W/in while the central area covered by the pan 67 has a much higherwatts density of about 75.6 W /in*. This explains the relatively smalltemperature gradient between about 600 F. of the central area and about750 F. of the outer uncovered area which of course results in lowthermal stresses. It also explains the high speed of the solid plateheating unit 10 of boiling a quart of water in the pan 67 in about 8.5minutes as seen in FIG. 33. There is a low current leakage capability ofthis solid plate surface heating unit 10 because of the relatively lowoperating temperature. Moreover the sheath temperatures are onlyslightly higher than the temperature of the top plate. One alternativewould be to reduce the spacing between the two outermost coils over thespacing at the center of the unit 10 so as to compensate for the heatlosses at the periphery of the unit and thereof obtain uniformtemperature distribution.

Turning to a consideration of standard spiral coiled heating element 65of FIG. 4A which is of the same wattage as the heating element 18 ofFIG. 3A With the standard spiral coiled heating element 65 the outeruncovered area runs relatively hot at about 39 W/in while the centralarea covered by the pan 67 is of equal watts density 39 W/in Thisresults in a rather large temperature gradient between the centralcovered area of 800 F. and the outer uncovered area of l450 F., which isthe temperature condition at the time of reading boiling temperature ofa quart of water after 13 minutes of heatmg.

In summary, it is clear that the use of the light weight composite sheetmaterial for the top plate 12 of the heating unit renders the heatenergy of the unit automatically adjustable with a fast response to thesize of the cooking utensil.

A relative performance chart of two 1250 .W. surface units of thestandard coiled design and the solid plate design of the presentinvention is given below:

The weight of the heating element 18 of the present invention is aboutone-half of the weight of a standard heating element 65 of exactly thesame wattage. This is possible because of the use of the top plate 12 ofhigh thermal conductivity for fast head distribution and less heat lossas is clear from a comparison of FIGS. 3A and 4A The thermal mass of aheating unit is a direction function of the weight and the specific heatof the unit. Since the solid plate unit 10 operates at lowertemperatures for the same amount of heating, less heat is stored in theunit resulting in fast response to both heat-up and cool down. This iscalculated by the formula:

where Q= Stored Heat W= weight in lbs.

At temperature change from room ambient to operating temperature Asecond modification of the present invention is shown in cross-sectionalelevational view in FIG. 6, wherein the same elements used in the firstmodification will be given the same reference numerals. This secondmodification is identified as solid plate surface heating unit 50 havinga top plate 12 of composite sheet material with'the center core 14 ofhigh thermal conductivity and the top and bottom outer skins 15 and 16respectively of stainless steel or the like, where the peripheral edgesof the skins l5 and 16 are sealed over the peripheral edge of the centercore 14.

A different type of coiled metal sheathed resistance heating element isused; namely, one having a generally clover leaf configuration 52, as isbest seen in FIG. 5, for covering as much as possible of the surfacearea of the top plate 12 on the underside thereof. Another departurefrom the first modification is to provide an integral reflector pan 54which is provided with generally radially ribs 55 and 56 which arelocated between the coils of the heating element 52 and are formedupwardly so as to engage the underside of the solid plate 12 as isbestseen in FIG. 4. The reflector pan 52 is dished out in the vicinity ofthe coils of the heating element 52 as is seen at 58 and 59 in FIG. 6.Thus it will be seen that I have combined two functions that are presentin the first modification of FIG. 2; namely, that of the reinforcingmember 22 and the reflector member 35. In the second modification thereflector 54 satisfies both functions.

The metal sheathed resistance heating element 52 is also a flattenedmember that is fastened as by brazing along its length to the undersideof the solid plate 12. The bottom surface of this plate and heatingelement assembly may have a low emissivity metallic finish. Thereflector 54 is made of a low thermal conductivity, thin stock materialsuch as stainless steel or the like. The upper surface of this reflector54 will have low emissivity. In order to prevent the oxidation of thereflective finish, the reflector 54 is hermetically sealed at itsperiphery to the underside of the plate 12 and the space between thesolid plate 12 and the reflector 54 is evacuated. Accordingly, thereflector surfaces will stay reflective during the life of the unit.

Modifications of this invention will occur to those skilled in this art.Therefore it is to be understood that this invention is not limited tothe particular embodiments disclosed but that it intended to cover allmodifications which are in the true spirit and scope of this inventionas claimed.

lclaim:

l. A low thermal mass, plate surface heating unit comprising a lightweight top plate of high termal conductivity material, a metal sheathedelectrical resistance heating element held in contact with a wide areaof the underside of the plate to obtain generally uniform heatingwhereby the top plate acts as a heat sink for the heating element, thetop plate being formed of composite sheet material with a center core ofhigh thermal conductivity selected from the group comprising copper,silver and aluminum, and an outer skin sealing the core and selectedfrom the group of materials comprising stainless steel, nickel andchromium, said outer skin serving to reinforce the center core andprevent its oxidation and corrosion, and a light weight reinforcingmember of large depth braced against the underside of the plate acrossthe span thereof to prevent warpage of the plate due to uneventemperature distribution.

2. A low thermal mass, plate surface heating unit as recited in claim 1wherein the said light weight reinforcing member is of open frameworkdesign having a series of vertically disposed radial struts extendingoutwardly from a center member, the top edges of the radial struts beingattached to the underside of the top plate.

3. A low thermal mass, plate surface heating unit as recited in claim 1wherein the heating element is of coiled configuration, and the lightweight reinforcing member is a reflector pan-shapedmember whichunderlies the coiled heating element and has embossed ribs interposedbetween the coils of the heating element, and these ribs are attached tothe under side of the top plate. oxidation 4. A low thermal mass, platesurface heating unit as recited in claim 3 wherein the underside of thetop plate and the heating element has a low emissivity coating, whilethe reflector pan is formed of thin metal stock of low conductivity, theperiphery of the pan being hermetically sealed to the underside of thetop plate, while the interior of the reflector pan also has a lowemissivity reflective coating, and the pan interior is evacuated inorder to prevent the oxidation of the reflective surface.

v 5. A low thermal mass, plate surface heating unit comprising a lightweight top plate of composite sheet material with a center core of highthermal conductivity selected from the group of high thermalconductivity metals and alloys such as copper, silver and aluminum andanouter skin surrounding the center core and selected from the group ofoxidation and corrosion resisting metals and alloys such as stainlesssteel, nickel and chromium, a metal sheathed electrical resistanceheating element of coiled configuration fastened to a wide .area of theunderside of the plate to obtain generally uniform center core selectedfrom the group of high thermal conduc' tivity metals or alloys such ascopper, silver and aluminum, and an outer skin selected from the groupof strong and oxidation and corrosion resisting metals or alloys such asstainless steel, nickel and chromium, a metal sheathed electricalfesistance heating element of coiled configuration fastened theunderside of the plate over a wide area to obtain generally ing a thinplate formed as an inverted pan with downturned peripheral flange, theplate being of composite sheet material having a center core selectedfrom the group of high thermal conductivity metals and alloys such ascopper, silver and aluminum and an outer skin sealed over the core andselected from the group of oxidation and corrosion resisting metals andalloys such as stainless steel, nickel and chromium, a metal sheathedresistance heating element of coiled configuration attached to theunderside of the top plate over a wide area to obtain generally uniform,heating whereby the top plate acts as a heat sink for the heatingelement, and a light weight reinforcing member of large depth attachedto the underside of the top plate to prevent warpage of the plate due touneven temperature distribution.

8. A low thermal mass, plate surface heating unit as recited in claim 7wherein the coiled heating element is of generally multiple-leaf cloverconfiguration was to cover a substantial area of the thin plate, thesaid reinforcing member being in the form of a reflector pan whichcovers the heating element and has radial embossed ribs interposedbetween the coiled heating element and these ribs are attached to theunderside of the thin plate.

9. A solid plate surface heating unit comprising a light weight topplate of composite sheet material with an inner core of high thermalconductivity selected from the group comprising copper, silver andaluminum, the core being sealed in an outer skin selected from the groupcomprising stainless steel, nickel and chromium, the said top platebeing substantially flat with a downturned peripheral flange toreinforce the plate, a spiral coiled metal sheathed resistance heatingelement fastened along the length thereof to the underside of the topplate over a wide area to obtain generally uniform, heating whereby thetop plate acts as a heat sink for the heating element, additionalreinforcing means of light weight open framework and large depth alsofastened to the underside of the top plate to insure that the plate willremain in a flat condition when it is operating at hightemperatures inthe range up to about l400 F.

10. A solid plate surface heating unit comprising a light weight topplate of composite sheet material with an inner sheet of high thermalconductivity on the order of copper and a top and a bottom sheet oflower conductivity but of higher strength and resistance to oxidationthan the inner sheet on the order of stainless steel, the peripheraledges of the top and bottom sheets being sealed together over the edgeof the inner sheet to protect the inner sheet against corrosion andoxidation, a spiral coiled metal sheathed resistance heating elementfastened to the underside of the top plate over a wide area to obtaingenerally uniform, heating whereby the top plate acts as a heat sink forthe heating element, and a light weight reinforcing means of large depthalso fastened to the underside of the top plate to insure that the platewill remain in a flat condition when it is operating at temperaturesreaching as high as 1400 F.

11. A solid plate surface heating unit as recited in claim wherein thesaid reinforcing member is of open framework member which isadapted tounderlie the edge of an openin in a cooktop in WhlC the umt is to bemounted, and an a 10 justable tension member joining the reinforcingmember to the reflecting member for clamping the member in a stationaryoperating position.

13. A cooking appliance comprising a cooktop having at least one openingformed therein, a solid plate surface heating unit mounted in the saidopening, the unit having a light weight solid plate of composite sheetmaterial of relatively high thermal conductivity positioned over theopening, the peripheral edge of the plate being formed downwardly into areinforcing flange, a spiral coiled metal sheathed resistance heatingelement fastened along the length thereof to the underside of the topplate over a wide area to obtain generally uniform, heating whereby thetop plate acts as a heat sink for the heating element, a light weightreinforcing member of large depth also fastened to theunderside of thetop plate to prevent warpage of the plate and projecting down throughthe cooktop opening, a thermal reflecting member underlying the unit forredirecting the heat in an o utward direction, said reflecting memberunderlying the edge of the opening in the cooktop in which the unit ismounted, and an adjustable tension member joining the said reinforcingmember and the said reflecting member so that the edge of the opening isclamped by the interaction of the top plate and the reflecting member.

14. A surface heating unit for a cooking utensil defining a containerfor the preparation of food, wherein the unit is a low thermal mass,solid plate surface heating unit that has a light weight top plate ofcomposite thin sheet metal material with a high thermal conductivity andan exterior surface that is resistant to oxidation and corrosion, and aspiral coiled metal sheathed resistance heating means of about halfstandard weight for a given wattage fastened to the underside of the topplate along the length thereof over a wide area to obtain generallyuniform, heating whereby the top plate acts as a heat sink for theheating element, a light weight reinforcing means of large depthfastened to the underside of the top plate to resist warpage of theplate during operation at high temperatures due to thermal stresses, theheat flow of the top plate of the unit being automatically adjustable tothe size of the bottom face of the utensil to deliver substantially moreheat to the area of the plate in contact with the bottom of the utensilthan to the outermost exposed area of the plate, the outer coils of theheating element being closer spaced than the center coils so as tocompensate for the heat losses at the periphery of the plate and obtaingenerally uniform temperature distribution across the plate when it issubstantially completely covered by a cooking utensil, the conductivityof the top plate being of such a high value that it operates at atemperature only slightly higher than the temperature of the sheath ofthe heating element which permits the use of a lower temperature heatingelement of about half the weight over standard open coiled units ofsubstantially equal wattage, while the relatively low operatingtemperatures of the sheath of the heating element also results ingreatly reducing the current leakage through the heating element to thetop plate and thence to the supporting structure for the heating unit.

1. A low thermal mass, plate surface heating unit comprising a light weight top plate of high termal conductivity material, a metal sheathed electrical resistance heating elEment held in contact with a wide area of the underside of the plate to obtain generally uniform heating whereby the top plate acts as a heat sink for the heating element, the top plate being formed of composite sheet material with a center core of high thermal conductivity selected from the group comprising copper, silver and aluminum, and an outer skin sealing the core and selected from the group of materials comprising stainless steel, nickel and chromium, said outer skin serving to reinforce the center core and prevent its oxidation and corrosion, and a light weight reinforcing member of large depth braced against the underside of the plate across the span thereof to prevent warpage of the plate due to uneven temperature distribution.
 2. A low thermal mass, plate surface heating unit as recited in claim 1 wherein the said light weight reinforcing member is of open framework design having a series of vertically disposed radial struts extending outwardly from a center member, the top edges of the radial struts being attached to the underside of the top plate.
 3. A low thermal mass, plate surface heating unit as recited in claim 1 wherein the heating element is of coiled configuration, and the light weight reinforcing member is a reflector pan-shaped member which underlies the coiled heating element and has embossed ribs interposed between the coils of the heating element, and these ribs are attached to the underside of the top plate. oxidation
 4. A low thermal mass, plate surface heating unit as recited in claim 3 wherein the underside of the top plate and the heating element has a low emissivity coating, while the reflector pan is formed of thin metal stock of low conductivity, the periphery of the pan being hermetically sealed to the underside of the top plate, while the interior of the reflector pan also has a low emissivity reflective coating, and the pan interior is evacuated in order to prevent the oxidation of the reflective surface.
 5. A low thermal mass, plate surface heating unit comprising a light weight top plate of composite sheet material with a center core of high thermal conductivity selected from the group of high thermal conductivity metals and alloys such as copper, silver and aluminum and an outer skin surrounding the center core and selected from the group of oxidation and corrosion resisting metals and alloys such as stainless steel, nickel and chromium, a metal sheathed electrical resistance heating element of coiled configuration fastened to a wide area of the underside of the plate to obtain generally uniform heating whereby the top plate acts as a heat sink for the heating element, and a light weight reinforcing member of open framework and large depth attached to the underside of the plate to prevent warpage of the plate, said framework having a central member and a series of radial struts extending outwardly therefrom.
 6. A low thermal mass, plate surface heating unit comprising a light weight plate of composite sheet material with a center core selected from the group of high thermal conductivity metals or alloys such as copper, silver and aluminum, and an outer skin selected from the group of strong and oxidation and corrosion resisting metals or alloys such as stainless steel, nickel and chromium, a metal sheathed electrical resistance heating element of coiled configuration fastened to the underside of the plate over a wide area to obtain generally uniform, heating whereby the top plate acts as a heat sink for the heating element, and a light weight reinforcing member in the form of a reflector pan which underlies the heating element and is also attached to the underside of the plate to prevent warpage of the plate, the underside of the top plate and the heating element having a low emissivity coating, while at least the interior of the reflector pan has a reflective coating.
 7. A low thermal mass, plate surface heating unit comprising a thin plate formed as an inverted pan with downturned peripheral flange, the plate beIng of composite sheet material having a center core selected from the group of high thermal conductivity metals and alloys such as copper, silver and aluminum and an outer skin sealed over the core and selected from the group of oxidation and corrosion resisting metals and alloys such as stainless steel, nickel and chromium, a metal sheathed resistance heating element of coiled configuration attached to the underside of the top plate over a wide area to obtain generally uniform, heating whereby the top plate acts as a heat sink for the heating element, and a light weight reinforcing member of large depth attached to the underside of the top plate to prevent warpage of the plate due to uneven temperature distribution.
 8. A low thermal mass, plate surface heating unit as recited in claim 7 wherein the coiled heating element is of generally multiple-leaf clover configuration so as to cover a substantial area of the thin plate, the said reinforcing member being in the form of a reflector pan which covers the heating element and has radial embossed ribs interposed between the coiled heating element and these ribs are attached to the underside of the thin plate.
 9. A solid plate surface heating unit comprising a light weight top plate of composite sheet material with an inner core of high thermal conductivity selected from the group comprising copper, silver and aluminum, the core being sealed in an outer skin selected from the group comprising stainless steel, nickel and chromium, the said top plate being substantially flat with a downturned peripheral flange to reinforce the plate, a spiral coiled metal sheathed resistance heating element fastened along the length thereof to the underside of the top plate over a wide area to obtain generally uniform, heating whereby the top plate acts as a heat sink for the heating element, additional reinforcing means of light weight open framework and large depth also fastened to the underside of the top plate to insure that the plate will remain in a flat condition when it is operating at high temperatures in the range up to about 1400* F.
 10. A solid plate surface heating unit comprising a light weight top plate of composite sheet material with an inner sheet of high thermal conductivity on the order of copper and a top and a bottom sheet of lower conductivity but of higher strength and resistance to oxidation than the inner sheet on the order of stainless steel, the peripheral edges of the top and bottom sheets being sealed together over the edge of the inner sheet to protect the inner sheet against corrosion and oxidation, a spiral coiled metal sheathed resistance heating element fastened to the underside of the top plate over a wide area to obtain generally uniform, heating whereby the top plate acts as a heat sink for the heating element, and a light weight reinforcing means of large depth also fastened to the underside of the top plate to insure that the plate will remain in a flat condition when it is operating at temperatures reaching as high as 1400* F.
 11. A solid plate surface heating unit as recited in claim 10 wherein the said reinforcing member is of open framework construction having a central member with a series of outwardly directed vertically disposed arms that are attached at their upper edge to the underside of the top plate, and a thermal reflecting member positioned beneath the unit for redirecting the heat in an upward direction.
 12. A solid plate surface heating unit as recited in claim 11 wherein the said thermal reflecting member is also a holddown member which is adapted to underlie the edge of an opening in a cooktop in which the unit is to be mounted, and an adjustable tension member joining the reinforcing member to the reflecting member for clamping the member in a stationary operating position.
 13. A cooking appliance comprising a cooktop having at least one opening formed therein, a solid plate surface heating unit mounted in the said opening, the unit having a light weight solid plate of composite sheet material of relatively high thermal conductivity positioned over the opening, the peripheral edge of the plate being formed downwardly into a reinforcing flange, a spiral coiled metal sheathed resistance heating element fastened along the length thereof to the underside of the top plate over a wide area to obtain generally uniform, heating whereby the top plate acts as a heat sink for the heating element, a light weight reinforcing member of large depth also fastened to the underside of the top plate to prevent warpage of the plate and projecting down through the cooktop opening, a thermal reflecting member underlying the unit for redirecting the heat in an outward direction, said reflecting member underlying the edge of the opening in the cooktop in which the unit is mounted, and an adjustable tension member joining the said reinforcing member and the said reflecting member so that the edge of the opening is clamped by the interaction of the top plate and the reflecting member.
 14. A surface heating unit for a cooking utensil defining a container for the preparation of food, wherein the unit is a low thermal mass, solid plate surface heating unit that has a light weight top plate of composite thin sheet metal material with a high thermal conductivity and an exterior surface that is resistant to oxidation and corrosion, and a spiral coiled metal sheathed resistance heating means of about half standard weight for a given wattage fastened to the underside of the top plate along the length thereof over a wide area to obtain generally uniform, heating whereby the top plate acts as a heat sink for the heating element, a light weight reinforcing means of large depth fastened to the underside of the top plate to resist warpage of the plate during operation at high temperatures due to thermal stresses, the heat flow of the top plate of the unit being automatically adjustable to the size of the bottom face of the utensil to deliver substantially more heat to the area of the plate in contact with the bottom of the utensil than to the outermost exposed area of the plate, the outer coils of the heating element being closer spaced than the center coils so as to compensate for the heat losses at the periphery of the plate and obtain generally uniform temperature distribution across the plate when it is substantially completely covered by a cooking utensil, the conductivity of the top plate being of such a high value that it operates at a temperature only slightly higher than the temperature of the sheath of the heating element which permits the use of a lower temperature heating element of about half the weight over standard open coiled units of substantially equal wattage, while the relatively low operating temperatures of the sheath of the heating element also results in greatly reducing the current leakage through the heating element to the top plate and thence to the supporting structure for the heating unit. 